The player comes out of a building after a bit of silence in action. Possibly with companions. They would be greeted with a street full of corpses of rebel and civilians. Cover around the corpses are destroyed. Explosion crater displayed near the middle of the street (near most of the corpses).

As the player gets a bit closer, if there's a companion they would say something along the lines "Oh my god..Who would do such a thing?" He would take a few steps closer to a body, and a trap would activate (possibly a mine.) it would explode injuring or killing the companion (or injuring you) Them from the rooftops and high windows gun fire would rain down on the player. Some doors would explode and Combine Soldiers would pour outwards onto the street.

Once that was over it would be a race to escape their pursuit (as there would be too much for the player to handle. Giving the idea that the player isn't really an invincible killing machine.)

And that's my suggestion. I tried to keep it vague so it would fit into whatever story you guys had in place.

I'm stating, that if you want to play Half-Life MP and it's mods online, you have to remove the file off of the Half-Life folder, or else you'll be either logged by VAC, or told you cannot enter the server.